The invention relates to a circuit arrangement for comparing the synchronism or tracking between two analog signals. Such a circuit comprises two inputs to which the respective analog signals are applied for supplying these analog signals to a comparator which is connected to a switch for producing an error signal in response to a determined deviation between the two signals.
It is frequently necessary to check and monitor the synchronism or tracking between two or more analog signals or measured values which are dependent upon one another. Such checking and monitoring may be necessary in manufacturing operations, in the process technology, for example, in chemical engineering, and in control engineering generally.
If the signals to be monitored deviate from one another to an extent which falls outside a certain tolerance frame, a switch or switching circuit or device makes sure that either the entire operation that is to be monitored is interrupted or that such operation is returned automatically to the desired rated value.
Thus, for example, in connection with transit systems, cabin taxis, and similar systems the fully automatic operations of the vehicle make it necessary to monitor the operation since the vehicles travel without an operator. For this purpose, for example, the speed and the spacing or distance between two vehicles one of which follows the other, are constantly measured and monitored to prevent a collision of the vehicles. Such a monitoring must provide an inherent safety feature. Stated differently, it must be assured that a collision of the vehicles cannot occur in fact if a failure should occur or if the used circuits include a source of error. To provide this assurance, it is customary to ascertain and monitor each value that must be monitored in at least two channels in order to be able to ascertain and process the correct data at least through one channel if the other channel fails or is subject to a defect.
In addition to the above mentioned problem of achieving the fail-safe feature, there is the problem that the measured data or values involved cover a large dynamic range. Thus, when monitoring the speed of a vehicle in a cabin taxi system, the output signal, for instance of a tacho-generator may vary between a zero value and 10 volts. Such a range is monitored between 100 millivolts corresponding to the smallest speed yet to be taken into account and 10 volts corresponding to the maximum speed. The above mentioned type of circuit arrangement must make it possible to ascertain whether the synchronism or tracking between the two analog signals is still present within the entire just mentioned range. This type of requirement calls for a complex circuit structure which must be tuned so to speak to the individual dynamic ranges.
In addition, it must be taken into account that the two analog signals normally are permitted to differ within relative and absolute tolerance limits. Such tolerance limits differ depending on the type of monitoring. These limits are determined by the special dimensioning of the circuit arrangement.